This invention relates to tufting machines and more particularly to tufting machines having A.C. motor drives and means for controlling the starting and stopping of the tufting machine mainshaft so that the needle bar may be gradually reciprocated when the machine is started and may be gradually stopped and when stopped positioned at the top of its stroke.
Tufting machines include a rotatable mainshaft which carries a plurality of drive members including those for reciprocably driving a needle bar carrying a plurality of needles and for oscillating the loop seizing members which cooperate with the needles to form stitches. The mainshaft is rotatably driven by one or two electric motors which conventionally may be substantially fixed speed A.C. motors, or variable speed A.C. or D.C. motors. Selection of the type of motor drive is a matter of preference dependent upon a number of factors, and although some of these factors are subjective, cost enters into the selection process. Since A.C. motor drives without variable speed features are somewhat less expensive than the other drives, a substantial number of tufting machines utilize these motors.
A difficulty exists in the tufting process when it is desired to stop the machine, as for example, to thread those needles wherein the thread may have been broken or otherwise unthreaded from the needles. In order to thread the needles, they must be in a raised position above the fabric being tufted, and because threading is most convenient when the needles are at the uppermost portion of the needle bar stroke, it is highly desirable to be able to stop the rotation of the mainshaft when the needle bar is at the top of its stroke. Additionally, and more critical to the appearance of the carpet product being manufactured, particularly loop pile fabric, is that when the machine is stopped rapidly and started rapidly, and especially when stopped at varying locations above the fabric, a distinct line or lines will appear in the fabric which is known in the art as "stop marks." Such "stop marks" are less apparent in cut pile fabric, but in the case of loop pile fabric, it can result in defective product resulting in waste or reduced quality product.
Although "stop marks" are a relatively small problem when the more expensive variable speed A.C. motors or variable speed D.C. motors are used, it remains a difficulty in those situations. When variable speed A.C. motors are used, the motors and thus the mainshaft can be slowed gradually and when the shaft has reached a predetermined speed, a speed sensor actuates a relay to engage a brake to stop rotation of the shaft. The motors and thus the mainshaft can subsequently be restarted gradually. Variable speed D.C. motors are even more expensive than the variable speed A.C. motors, but are less reliable and thus less popular.
The most significant "stop marks" problem results when using the popular constant speed A.C. motors, i.e., induction motors. Tufting machines driven by these motors utilize a single disk brake, air actuated through a solenoid valve receiving an electrical signal when the motor stop button is depressed. However, the motor and thus the mainshaft is rapidly brought to an uncontrolled stop and the needle bar may come to rest randomly at any position. Thereafter, the machine operator uses the "jog" button to position the needle bar at or close to the top of the needle stroke and then may turn a large wheel on the end of the mainshaft to adjust the position of the needle bar at the top of the stroke. When the machine is thereafter started, and the needles form the first stitches, "stop marks" inherently result. One prior art attempt to solve this problem is described in Owens U.S. Pat. No. 3,753,061 wherein a D.C. voltage is applied to the windings the A.C. motors for stopping the motors after the stop button has been depressed. For various reasons, however, this proposal does not appear to have been adapted, and in any event, is not known to now be used.